Second Form:When a character dies you kind of have to wing it. If you're playing cautiously you need to do whatever it takes to make sure that the dead character is revived, healed, and hasted. Remember, an item takes less time than casting curaga so even if it's yuna's turn in an emergency she should still use items to heal so her next turn comes quicker. I usually just revived the character and then went on with the usual battle plan, keeping wakka attacking and only healing with the other characters.

Second Form:Before you kill jecht everybody needs to have protect on them. Preferably only go in with full health, Regen, and protect on everybody. So get him down to like 5,000 or however much wakka can do it one hit and then only attack when these conditions are met. Use Titus' talk command to keep his overdrive down but keep your eye on the turn gauge to make know when to use it.

Stoneproof (20x Petrify Grenades) on your armors to make is much easier to deal with Jecht Beam. Zombieproof (10x Candles of Life) to deal with the zombification from Triumphant Grasp in the first form. Then add anything else you feel like adding.

Get the Petrify Grenades by capturing one of each Monster in Djose Highroad (Don't forget the Ochu's on the MoonFlow). The Candles of Life by capturing one of each Monster in Mushroom Rock road. You'll get 99 of each, enough to customize 4 armors of Stoneproof, and 9 armors of Zombieproof. Steal one more Petrify Grenades from a Basilisk to make one more armor with Stoneproof if you want. If you want, you can use 30 of the Candles of Life to customize zombiestrike on Auron's weapon to help induce zombie and stop the healing of the pagodas.You'll also get 60x Three Stars after unlocking those creations. Use those to customize One MP Cost on Lulu's and Yuna's weapons. You'll still have enough of them to customize One MP cost on one more weapon. I'd say either Tidus (so he can use Quick Hit over and over) or Auron (so he can use Power Break over and over).

Also, capture one of each Monster in Besaid (there's only three of them) to get 99x Stamina Tonics.

Now, go Bribe a Grendel (found in Gagazet Cave/Road to Zanarkand) for Mega-Potions.

As for the battle itself:

Have Tidus cast Hastega. Switch Rikku in (haste her too) and have her Use A Stamina Tonic on the party to double their Max HP. This lasts till they're killed (which shouldn't happen). Now, switch your people around and throw Stamina Tonics till you have Double HP and Haste on all the people you want to use. Now, Bring in your main party, and Have Tidus use His Cheer special five times. This will reduce the damage from BFA's normal attack, overdrives, and Sword swipe by 33%. Cast Protect on your frontline party as well. Meanwhile, Auron just uses Power Break over and over to keep the damage from BFA's normal attack at bay. When BFA's nearing OD, make sure that you've already double'D your HP and casted Cheer 5 times. now, it'll not kill anyone, unless you're really underleveled.

From now on, just unleash whatever attacks you want on him. Haste + Cheer *5 + Protect + Double HP will make sure it's VERY hard for you to die, unless you forget to heal from some reason or another. Now, you've got your Mega-Potions. Those will heal you for 2000 HP a pop (all frontline party), use those to heal when you need to.

In the second form, he'll start using Sword Swipe, but, you shouldn't have a problem keeping up with it. Heal with mega Potions and keep on the assault. Triumphant Grasp, and Ultima Jecht Shot shouldn't be able to kill anyone , so just keep an eye for healing and hack away.

Holy, Double Cast Flare, Auron's Attack (if you gave him One MP Cost have him use Power Break instead of his normal attack), Quick Hit (Again, if you gave him One MP cost). Overdrives, anything you want.